3D printing large surfboard fins

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Yes, wet sanding is the best way. Under water works best, but the material is quite porous and will soak up a lot of water.
I whittle off the support system after printing and dry-sand just a little bit to get rid of sharp edges, then fill the fin with resin. When the resin has set, I sand the fin under water, then let it dry, then coat it with resin.

I have not yet tried the gullwhale fin, but the Harftub7s13, is basically my all time favorite fin in my 9'7" traditional style longboard. It is a bit stiffer and more stable than the harftub9.5 and not quite as forgiving, but this seems to complement my board better, and I have had many supremely satisfying cutbacks at speed, where I kick out of the wave laughing.

I'll never go back to a traditional shaped fin in my longboards again, unless it is a kelp/seaweed fest.

Effectively Sanding the PLA ridges is kind of a pain. I've started using my machine sander with wetted 220 to knock them down, then hand wetsand with 3M wet dry 120grit, then 220, then spraying rattlecan laquer. The razor blade scraping along the ridges also works pretty well, but if epoxy covered already, I'd just wetsand to 400, if that.

More swaylockians should be bugging MrMik to try these fins. I've lent a 9.5 to a highly skilled local surfer who rode it in a sub 7 foot single fin, who was amazed at the speed but said it turned a bit weird. With Such a deep fin(9.5") on a sub 7 foot board, I did not expect to get a 'speedy' description.

I have not yet tried the gullwhale fin, but the Harftub7s13, is basically my all time favorite fin in my 9'7" traditional style longboard. It is a bit stiffer and more stable than the harftub9.5 and not quite as forgiving, but this seems to complement my board better, and I have had many supremely satisfying cutbacks at speed, where I kick out of the wave laughing.

I'll never go back to a traditional shaped fin in my longboards again, unless it is a kelp/seaweed fest.

Effectively Sanding the PLA ridges is kind of a pain. I've started using my machine sander with wetted 220 to knock them down, then hand wetsand with 3M wet dry 120grit, then 220, then spraying rattlecan laquer. The razor blade scraping along the ridges also works pretty well, but if epoxy covered already, I'd just wetsand to 400, if that.

More swaylockians should be bugging MrMik to try these fins. I've lent a 9.5 to a highly skilled local surfer who rode it in a sub 7 foot single fin, who was amazed at the speed but said it turned a bit weird. With Such a deep fin(9.5") on a sub 7 foot board, I did not expect to get a 'speedy' description.

I know that you sand the ridges to reduce the tendency to hold on to seaweed, but have you noticed any effect on the fins performance if you sand and coat the fins to be more smooth? It might make them perform better or worse, I have no idea if and what difference it makes. I know it makes a lot of difference to the amount of manual work that goes into each fin.
I could also print them with reduced layer height, I could at least half it, but that will double the printing time and possibly reduce the mechanical strength. However, I'm hopeful that my method of combining PLA and epoxy results in increased strength, possibly independently of printing layer heights.
A few earlier attempts to use differential layer heights for printing the tubercles much smoother have had poor results, but I think I was trying to push it too far.
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The HARFTUB-9 that snapped off was used in a 5'4'' x 22 1/4'' x 3 1/8'' (or very similar) board and got very good ride reports. I'm waiting to hear how the HARFTUB-7-S-13mm goes in the small boards and have a GullWhale-7-S-11mm ready for that surfer to try in his small boards.

The fin does feel more draggy than the WG2 of exactly the same length. However, for a fin almost twice the thickness it doesn't feel as draggy as you might expect - not too bad at all.
In terms of turning... the fin feels pretty solid but I think it might have felt a lot better half an inch forward of centre towards the nose. I really struggle to get any hard turn on the very small waves but I did at least get 2 real bottom turns in. Blame the rider but I couldn't find a top to top turn off!

The fin feels slightly weird. The feeling of turning is slightly before what I expect and I'm used to. Having thought about it for a while I guess this is because the gullwhale bends forwards. If so then I'm amazed I can feel this in such small conditions. I wonder if in bigger conditions that feeling would throw my timing a bit and I just would get so hard leaning over in the bottom turn.

That's all I can really say for now. The waves were just too small to really test anything more than this today. I like the fin but clearly it's not the right choice for today. I'd choose it in bigger waves.

Anything else?

When the fins arrived I instantly like that the PLA is both usable but at the same time it's not as good as fibreglass. In this sense it's great for testing and inspires me to resume my printing attempts. But at the same time it's not really a threat for fin makers yet.

I forgot that the fins are design to just squeeze into a finbox and stupidly drilled a hole for the retainer. While this was silly it gave me a really good sense of the strength of the fin. All the thickness is needed for the strength and I don't think this is going to work for shortboard fins. Well, that's the sense I get.

I actually scraped a rock while sitting in the lineup. The fin just took a few scratches. Not bad.

It would have been nice to hot swap the fins while in the water but to cut a long wetsuit story short I couldn't really do it this time.

Thanks for the feedback!
It made me realise that I stuffed up. Trying to keep too many ball in the air juggling.....
The GullWhale 7 S should not be 13mm thick, it should be 11mm thick (well, really 10.7mm but I decided to be pragmatic, considering I don't really know how to determine best thickness).
So you are the proud owner of an extra-thick fin.
At least the HARFTUB-7-S-13mm is as thick as it should be.
Somehow, in the hustle and bustle, I changed the GullWhale fin thickness to 13mm. I think the fin you have is most likely actually 13mm thick.
You can measure them at the base, thickest part should be as thick as the number printed on the side of the fin base. Give or take 1/3rd of a mm or so.
I have thought about the question if the fin size determines strength at the likely snap-off line (just above the fin box). So far, I think that the relationship between fin area (which I think is linearly proportional to the maximum force exerted on the fin by the water) and the size of the coronal section through the area that snaps off is constant, when you shrink or enlarge a fin. So my assumption is that a miniature version of a fin will not snap more easily than a large version of the same fin. I cannot prove it, or even convince myself conclusively, but that's my 'gut feeling' for now.

And your feed back has also given me a good idea about how to improve the thickness distribution, so that thinner fins do not end up being much weaker at the base connection to the tab:
A steep flange, which is cut off at the base, so that there is actually not much of a flange, but the attachment area at the base is larger.
It means re-rendering at high resolution is required and it will take some time before I get around to it and can ask Hans to render finFoil files for me.

The HarfTub 7s is 13mm
and
the Gulwhale 7s is 11mm
^ and I confirmed that with a pair of callipers.

Will the wider Harftub 7s be slightly less draggy? Well, it's interesting that I really couldn't tell until I got them into a familiar board. It really has to be a case of getting some better waves before I can confirm. I've read that thicker fins can conversely be lower drag but somehow I still can't accept that as really I'm expecting that to be a detail of foil.

edit:
Regards smaller fins....
maybe they'll be OK but the thing that's screwed my efforts in the past was weakness between the lines of plastic. In a smaller fin those fins are comparatively bigger. Also, for many fin systems the actual base is smaller.
Definitely have to try it out. I printed off a 5 FCS tab fins with winglets. The first 2 snapped off on the first wave but those had a small void in the middle. I have some solid print to try but no waves and no decent board to try them in except a foil board prototype.

The HarfTub 7s is 13mm
and
the Gulwhale 7s is 11mm
^ and I confirmed that with a pair of callipers.

Will the wider Harftub 7s be slightly less draggy? Well, it's interesting that I really couldn't tell until I got them into a familiar board. It really has to be a case of getting some better waves before I can confirm. I've read that thicker fins can conversely be lower drag but somehow I still can't accept that as really I'm expecting that to be a detail of foil.

edit:
Regards smaller fins....
maybe they'll be OK but the thing that's screwed my efforts in the past was weakness between the lines of plastic. In a smaller fin those fins are comparatively bigger. Also, for many fin systems the actual base is smaller.
Definitely have to try it out. I printed off a 5 FCS tab fins with winglets. The first 2 snapped off on the first wave but those had a small void in the middle. I have some solid print to try but no waves and no decent board to try them in except a foil board prototype.

Good that the fins are the size they should be. Not good that I am loosing the plot here. Too many fins! I looked up my records yesterday and thought I saw 13mm, but that was not your fin. I did indeed print some GullWhale-S-13mm fins.
The GullWhale-7-S-11mm does not have a thicker foil profile than the HARFTUB-7-S-13mm. It is simply narrower, or higher aspect ratio than the HARFTUB-7-S-13mm. Relative to the chord length, the thickness is the same for both fins. At least that's what I was trying to achieve. Hard to be certain with such a complex shape. You can measure chord length from the peaks or from the valleys, but the maximum thickness is changing linearly along the wingspan. Therefore I measured at peaks and valleys, and then adjusted the thickness, aiming to make both peak and valleys fall into a good range of proven foil thicknesses.